After the Thaw - The Development of Lake Michigan; Page 2

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Geologic Time & Ancient Environments > After the Thaw

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Where's the Beach?

About 5,500 years ago, the level of the water surface in Lake Michigan was about 23 feet higher than today. At that time, the southern shore of Lake Michigan was in the early stages of forming the last of Indiana's shorelines—the Toleston Beach. In fact it is still in the process of making the Toleston Beach, today. Most people think of the Toleston Beach as the "Indiana Dunes." The Indiana Dunes, however, is only the area of the coast where there are high dunes. This area primarily runs from east of Michigan City westward to a little past Ogden Dunes (see middle figure on previous page). West and south of Ogden Dunes, the Toleston Beach fans out into more than 100 individual beach ridges. These ridges mark the position of Lake Michigan as sand was added to the shoreline. They can be thought of as rings in a bathtub or glass of Guiness stout as the liquid falls. Each mark, therefore, is younger than the previous.

Let's look at the development of the Toleston Beach in two areas. First, by following the creation of the Indiana Dunes and Great Marsh in the eastern part of the Toleston Beach, and second, by looking at the addition of beach ridges in the western part.

Creating "da Dunes"

The development of what is now called the Indiana Dunes began while lake level was still rising from the Chippewa low. A hiatus or even a short-term fall during the rise about 6,500 years ago permitted a linear ridge of sand to accumulate along the Indiana shore (see diagram to the right). This barrier beach formed a little farther off the position of the modern shore and migrated landward through time. This migration was caused by storms that forced waves to overtop the early Toleston Beach and deposit sand and gravel behind the beach in a lagoon. The lagoon formed in the low area between the new Toleston Beach and the abandoned Calumet Beach. Not only did the waves bring sand and gravel into the lagoon, but mud and silt came along with the waves. These sediments would settle out of the water during fairweather conditions, creating layers mud and silt. Some of these deposits are exposed today along the shoreline at Mt. Baldy.

As the Toleston Beach continued to migrate landward, it grew in height with the addition of dune sands. Eventually, the Toleston Beach stabilized in one position when lake level stopped rising. The beach, at this time, was too high for waves to overtop it (see second image in cross section above). The lagoon landward of the beach began to dry up and to separate into individual ponds. These ponds had so little sediment coming into them that they precipitated calcium carbonate (marl). Precipitating marl was not hard to do, because waters draining the glacial highlands were probably saturated with respect to calcium carbonate and a calcium carbonate loving plant was around (Chara) to provide some additional input when it died.

The Toleston Beach and Great Marsh remained in this state until about 4,500 years ago. Lake level rapidly fell at this time to levels not unlike those observed in the historical record. This fall exposed large parts of the Toleston shore and significant quantities of sand were blown landward forming large Toleston Beach dunes. Many dunes were active at this time, migrating landward into Great Marsh. The Great Marsh was also changing. The marl ponds disappeared and vegetation colonized across the entire area between the Calumet and Toleston Beaches (see the third image in the cross section above). The widespread colonization of the Great Marsh probably also helped by a drainage divide that was forming under the growing Toleston Beach. This divide permitted water to flow landward into the Great Marsh, making the Great Marsh wetter than when the marl ponds were there.

By 2,000 years ago, The Great Marsh had taken on a form similar to today. Ditching and farming in the late 1800s and early 1900s has altered this form. Research in the Indiana Dunes National Lakeshore is looking for ways to restore parts of the Great Marsh to pre-settlement conditions and functions.

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